The present invention relates to an improved method and apparatus for tempering and bending glass. More particularly, the present invention relates to a method and apparatus for tempering and molding glass using a flexible bladder.
A conventional glass bending apparatus typically includes two rigid molds--a male mold and female mold--which are brought together to conform a heated sheet of glass positioned therebetween to the shape of the two molds during the press cycle of the bending process. Some bending methods may include a slump bending step, which is initiated before the press cycle begins. In such a step, the heated glass sheet may be held in a holder that includes a generally concave, upper support surface, formed by a formable material such as a stainless steel mesh sheet. Since the sheet of glass is heated to a temperature in the range of 1000 degrees Fahrenheit to 1250 degrees Fahrenheit and is, therefore, in a softened state, the glass sheet partially conforms to the concave shape of the mesh sheet. Thereafter, as the female mold is moved towards the male mold, the female mold presses against the edge portions of the heated sheet of glass and the male mold presses on the center of the heated sheet of glass to start the press cycle of the bending process. As the two molds are pressed together, the bearing contact and pressure exerted by the molds on the edge portions and in the center causes the molds to further bend the sheet of glass. As the sheet bends, the bearing contact and pressure from the female mold expands from the edge portions of the sheet inward until it meets with the bearing pressure from the male mold, somewhere between the center and the perimeter of the sheet. Consequently, as the bearing pressure expands inwardly from the perimeter of the glass sheet, buckles form at the periphery of the plate to relieve forces due to the foreshortening of the compression side of the glass. As the bending is increased the buckles extend toward the center. The magnitude of the buckles are the greatest at the edge of the glass sheet, diminishing toward the center of the sheet.
The larger the buckles and the further the buckles extend into the glass sheet, the smaller the useable area of the glass. This useable area is referred to as the "quality area." The size of the quality area directly impacts the value of the glass. The larger the quality area, the larger the size of the final product--the curved glass--and the greater the profit. In optical quality glass or plastic, the tolerance for distortions in the surface of the glass is extremely low. Hence the size of the quality area for optical glass is relatively small when conventional glass bending techniques are used.
In addition to the limitations on the quality of the glass produced by conventional glass bending techniques, there are numerous process controls. The male and female molds require accurate alignment. This step can take up to two hours. Furthermore, the two rigid molds of the conventional glass bending apparatus require substantially perfect alignment of the heated sheet of glass between the two molds. If the glass sheet is not properly aligned, the peripheral edges of the sheet rest in a non-horizontal plane within the female mold so that when the female mold is moved toward the male mold, the male mold will first make contact with a portion of the glass that is off-set from the center of the glass sheet. Therefore, the glass sheet will have a bend that is off-center and, more than likely, the glass sheet will exhibit exaggerated buckling at least on one side of the sheet. Moreover, the conventional glass bending apparatus requires the temperature of the heated sheet of glass to be controlled within about 5 degrees. Also, debris, such as broken pieces of glass, that may be inadvertently left in the molding apparatus could damage one or both of the molds during the bending process.
Conventional methods of toughening or tempering a sheet of glass include air blast tempering and contact tempering. Air blast tempering is achieved by blowing cold air streams onto a sheet of glass heated to a toughening temperature. During the toughening treatment, the glass sheet is either held in a vertical position by tongs, which grip the upper edge of the glass sheet, or the glass sheet rests in a horizontal orientation with its edges resting on the perimeter of a support or on an air cushion. In contrast, contact tempering is achieved by contacting a hot glass sheet with cooled surfaces of rigid pressing plates of a press. Contact tempering generally achieves lower deformations or distortions of the glass sheet than air toughening where the glass sheet is only held at an edge or on one face.
Bending and tempering of a glass sheet is typically performed in two separate steps. The bending step includes heating a sheet of glass and positioning the heated sheet of glass between two rigid molds to conform the glass sheet to the molds. After bending, the bent glass sheet is immediately transferred to a cooling area. Once in the cooling area, air is blasted onto the bent glass sheet to cool the exterior surfaces of the glass and thereby temper the glass. However, this type of apparatus is typically only suitable for tempering and bending glass sheets of 3 mm thick or greater. Thinner sheets of glass do not have sufficient heat capacity to hold the heat until the bending process is complete since thin sheets undergo rapid heat loss during bending due to the relatively cool bending tools and cooler ambient atmosphere in the bending area. As a result, the temperature of the glass sheet is decreased to a level below the minimum temperature at which adequate tempering can be affected. Rapid heat loss can be compensated for by overheating the thin sheet of glass prior to bending, but overheating tends to degrade the surface quality of the finish glass sheet which results from heat strains, pitting and the like. In some processes, the bending molds are positioned within the furnace to maintain the temperature of the thin sheet at the desired tempering temperature. However, this process restricts the types of mold material and impedes mold servicing.
An apparatus for contact tempering and bending a sheet of glass is disclosed in U.S. Pat. No. 4,826,522 to Benoit D'Iribarne and Paul Houang in which rigid cooling plates bend and cool the glass sheet simultaneously. The edges of the glass sheet are air tempered by compressed air, which is blown onto the edges during the contacting step. However, this apparatus is typically suited for glass sheets of 5 mm thickness or greater, and is not well suited for tempering thin sheets of glass. While contact tempering is generally capable of tempering thinner glass than air blast tempering and, generally, provides a better conformance to the design shape for convex and concave parts, contact tempering has heretofore not been suitable for tempering very thin glass sheets, on the order of 3 mm or less.
Consequently, there is a need for an improved method and apparatus for tempering and bending glass that is suitable for relatively thin sheets of glass, for example sheets of glass having a nominal thickness of 3 mm or less, or for example, sheets having a nominal thickness of 1.7 mm or less, sheets having nominal thickness of 1.1 mm or less as well as other thickness. Moreover, there is a need for an improved method and apparatus for tempering and bending glass that will increase the size of the quality zone of the tempered sheet of glass and reduce the number of process steps and inputs required to control the process, while at the same time providing more control and uniformity to the tempering and bending process.